The present invention relates to a process for the preparation of aromatic substances by ultraviolet (UV) irradiation in the presence of oxygen of an alcoholic extract which contains diterpenes and has been obtained from the surface resin of fresh tobacco plants or parts thereof or from the gum originating from tobacco flowers.
As is known, the surface resin of fresh tobacco plants contains diterpenes, in particular duvanes, which, as smoke aroma precursors, influence the tobacco aroma. Numerous processes for isolating these diterpenes from the surface resin are known, and with these processes undersired constituents, in particular, lipids, which are likewise present in the resin are separated off. The process at this type is disclosed, for example, in German Offenlegungsschrift No. 2,918,920.
After isolation and purification, the diterpene fraction obtained in this way is added to conditioned tobacco (tobacco which has been processed ready for use).
In the past, the properties of diterpenes, isolated from tobacco plants, as smoke aroma precursors have induced several workers in the field to concern themselves with clarification of the chemical structure of the diterpenes and their chemical reactions, in particular with regard to their photoreactions with a singlet oxygen. Cf. Acta Chemica Scandinavica 1979, pp. 437-442. In these investigations, the photo oxidation was carried out in the presence of sensitizers, for example, Rose Bengal. The oxidation involves absorption of UV light by the sensitizer which undergoes an electronic transition to the excited singlet state. Singlet oxygen than reacts with the diterpenes to give the oxidized product.